STAR, a Sprawl Tuned Autonomous Robot

STAR, A Sprawl Tuned Autonomous Robot

David Zarrouk1, Andrew Pullin2, Nick Kohut2, Ronald S. Fearing1

Abstract– This paper presents a six-legged, sprawl-tuned and that the sprawled posture is more energy efficient. Full et autonomous robot (STAR). This novel robot has a variable leg al. presented a first sprawled robot, SprawlHex [ 7], which sprawl angle in the transverse plane to adapt its stiffness, can adjust its sprawl angle, up to 20 degrees, in order to height, and leg-to-surface contact angle. The sprawl angle can experimentally compare to animal behavior. be varied from nearly positive 60 degrees to negative 90 degrees, enabling the robot to run in a planar configuration, upright, or inverted (see movie). STAR is fitted with spoke wheel-like legs which provide high electromechanical conversion efficiency and enable the robot to achieve legged performance over rough surfaces and obstacles, using a high sprawl angle, and nearly wheel-like performance over smooth surfaces for small sprawl angles. Our model and experiments show that the contact angle and normal contact forces are substantially reduced when the sprawl angle is low, and the velocity increases over smooth surfaces, with stable running at all velocities up to 5.2m/s and a Froude number of 9.8.

I. INTRODUCTION Figure 1. The sprawl-tuned autonomous robot has three motors, one for each side of the legs and a third motor which actuates the sprawl angle. Drawing inspiration from insects, miniature crawling robots possess substantial advantages over wheeled vehicles The sprawl-tuned autonomous robot (or STAR), presented for off-road locomotion, such as in caves and collapsed in this work, is an autonomous crawler that has a variable buildings, for reconnaissance and search and rescue sprawl angle to adapt its legs to different surfaces. Inspired purposes. Their low weight and cost allow their deployment by SprawlHex and Mini-Whegs, this design allows the robot in large numbers, independently or in swarms, to cover a to transform its locomotion mechanics from the sagittal plane large work area and increase the odds that some of the robots to the lateral plane and combines the advantages of both will succeed in performing a specific task. Some existing vertical and in-plane locomotion. At low sprawl angles, the examples of comparable robots can crawl at more than 5 vertical contact angle of the foot with the surface is reduced (and up to 15) body lengths per second, such as Mini-Whegs which minimizes collisions and reduces the uncontrolled [ 10], Dyna-RoACH [ 4], DASH [ 1], iSprawl [ 6], vertical dynamics, resulting in smooth operation of the robot OctoRoACH [ 11], RHex [ 3], and Sprawlita [ 2]. Since it is at all speeds. The sprawl angle can also be used to change difficult to implement active leg joints at this scale, a key the width and height of the robot in order to fit between, challenge in developing a high speed and highly above, or underneath obstacles. The characteristic length of maneuverable miniature crawler is the design of passive the robot is 12 cm and the weight is 73 grams including mechanical elements which contribute to the stability of the battery and control board for autonomous operation. To robot during locomotion, similar to insects [ 8][ 5]. The improve stability and energy consumption, the robot is fitted crawling mechanism can be approximated using the spring with radially spoked legs which can perform almost like loaded inverted pendulum model (SLIP) [ 14][ 15], which regular wheels for low sprawl angles and as legs for high describes the locomotion of insects in the sagittal plane. sprawl angles. In-plane or lateral models of locomotion which neglect In this paper, we first present the mechanical design of the vertical oscillation, but are useful for steering and yaw robot in Section II. In section III we present the analysis and stability, were investigated by Schmitt and Holmes [ 12] [ 13], design considerations of the robot and the effect of the Kukillaya and Holmes [ 9], and Seipel et al [ 16] who studied sprawl angle on locomotion. In Section IV we present the the dynamic locomotion model of cockroaches. The lateral experimental results of the robot being run over different leg spring model (LLS) shows that sprawled angle postures surface conditions. are stable both in the sagittal lateral plane at different speeds, II. MEHANICAL DESIGN

The primary design goal of STAR is to achieve a small, 1 Department of EECS, UC Berkeley, ([email protected]). low cost, fast, highly-maneuverable and stable platform 2 Department of ME, UC Berkeley.

which is adaptable to different surface conditions. These goals can be achieved by minimizing the collision of the legs with the ground through variable leg geometry and compliance, thus reducing the uncontrolled vertical dynamics.

A. Robot and leg design STAR has a rigid body core which holds the onboard battery and the control board. Each side of the robot has three-spoke legs with drive distributed from a single motor to all three legs. A constant mechanical 60o rotational phase offset between neighboring legs reduces the aerial phase and collision with the ground of the robot. Each of the three legs and their motor are fixed on the same rigid link which is attached to the robot through a rotational pin joint. The relative angle between the legs and the main body, as presented in Figure 2, forms the sprawl angle, which is defined as !=0 when the legs are coplanar with the ground. The positive sense of the sprawl angle is as shown. The Figure 3. STAR at different sprawl angles. a) Positive sprawl angle. b) Zero sprawl angles at both sides are actuated symmetrically sprawl angle. c) Negative sprawl angle. through a single motor and mechanism to insure identical sprawl on both sides.

Figure 2. Front view of sprawl robot.

The sprawl angle can be varied in the range (+60o, -90o), as shown in Figure 3, allowing the robot to continue running in the same direction even when upside down. Figure 4. Spoke leg wheels.

The spoke wheel legs are composed of three compliant legs Similar to OctoRoACH [ 11], the leg set on each of the two attached to a main hub (see Figure 4). The length of each sides are not phase synchronized, and STAR uses differential velocity for steering using a closed loop controller, where the spoke Lleg is 28 mm, and the tip of the leg has a circular o yaw rate of the robot is measured using the onboard MEMS shape spanning .solid=30 . The gap angle, .gap, between the edges of the leg spokes is 90o, which allows the tips of the gyro and the velocity of the motor is measured from the legs to reach up to 4cm high for crawling over obstacles. motor back EMF. C. Actuation and control Each set of legs is driven by a 7 mm brushed DC motor D. Manufacturing (Didel MK07-3.3), and a transmission with a ratio of 48:1. STAR is designed for rapid manufacturing; the body core, For high speed we used a 16:1 ratio and (Didel MK07-2.3) motor housing, spur gears, and legs are 3D printed using a motors. The high ratio ensures high torque output and steady Projet 3000 machine. The printer‘s accuracy is roughly velocity. The robot uses a 300mA-hr LiPo, 4V battery from 0.05mm. The robot is designed for easy assembly and simple Full River giving 30 minutes endurance when running at full part replacement, and the total mechanical assembly requires drive capacity. roughly 30 minutes.

moment Ileg. The vertical compliance is the combination in series of the bending, torsion, and shortening of the leg. If we assume that the legs are rigid along their length, the total normal compliance of the legs as a function of the sprawl angle is -1 1 ≈’EI kk=+∆÷3 (8) nr2 ∆÷ Lleg cos()r «◊Lleg which, in this case, shows that the normal compliance is a decreasing function of the sprawl angle only. Figure 8 Figure 9. Robot front view. Slightly negative sprawl angle causes the robot to run in the opposite direction without changing the rotation direction of presents the variation of the normal stiffness, divided by the the legs. stiffness at zero sprawl , as a function of the sprawl angle. At 75 degrees, the relative stiffness increases by four fold. A. Velocity and stability In order to investigate the locomotion performance of the robot as a function of its sprawl angle, we ran the robot over horizontal plywood and carpet surfaces and over a 10 degree incline. The input velocity profile was composed of acceleration (one second), constant velocity (three seconds) and deceleration (one second). The robot‘s nominal velocity (as if equipped with regular wheels) during the constant speed phase was roughly 0.6m/s. Figure 10 shows the

Figure 7. Showing the normal compliance due to bending and torsional average velocity of robot, extracted from the steady state compliance. stage, for different sprawl angles. Over horizontal wood surface and with a 15o sprawl angle, the performance is almost identical to that of wheels, but the velocity drops as a function of the sprawl angle. The velocity for upright legs (!=90o) was roughly one third smaller, which is similar to what was reported by Morrey et al [ 10]. When running on an incline, a substantial reduction of speed is observed at a sprawl angle of 45o and beyond, implying that the thrust force is substantially reduced due to increased collisions with the ground.

Figure 8. Normal stiffness kn, divided by the stiffness at kn(0) at !=0, as a function of the sprawl angle.

E. Switching the locomotion direction When the robot is configured to a slight negative sprawl angle (!=0), the thrust of the legs will reverse due to the legs making contact to the ground on the inside of the swing rather on the outside. With onboard actuated sprawl angle, Figure 10. Velocity of the robot as a function of the sprawl angle over three this mechanism could be used to spontaneously reverse different surfaces. direction or reverse thrust braking without changing the spin Figure 11 presents a top view of the trajectory of the robot direction of the legs. using closed-loop gyro steering control [ 11] over carpet as IV. RESULTS obtained from the Vicon system. For small sprawl angles, the robot is easily capable of traveling straight with practically In this section we present the experimental results of our no error. As the sprawl angle increases, the deviation from robot, with various sprawl angles, running over different straight becomes more significant due to increasing ground surface conditions, and performing various maneuvers. contact forces causing disturbances to lateral motion. The maximum heading deviation error for 75o and 90o sprawl angles is respectively 5 and 3.5 degrees.

B. Contact ground forces (!=90o), the robot can easily surmount obstacles larger than the spoke length, up to 35mm. Figure 13a and b show the In order to investigate the ground reaction forces we use a robot crawling both under and over a 30 mm high bridge 3D force plate sensor with a resolution of 0.01 N. The robot (whose thickness is 5 mm). was run at roughly 0.5m/s and the sprawl angle was varied from 15 to 90 degrees. Figure 12 presents the average of the instantaneous peak normal forces due to the contact of the legs with the ground. Comparing the two extreme configurations, the normal force is measured to be five times larger at a sprawl angle of 90o than 15o.

Figure 13. a) The robot is seen crawling underneath the bridge. b) The robot crawling over the top of the same bridge.

2) High speed running and turning performance During all of our experiments, we used a transmission gear ratio of 48:1. STAR is designed to easily switch to ratios of 24:1 and 16:1 for higher maximum speeds. At low sprawl angle, the locomotion was highly stable and the robot attained 5.2 m/s in straight locomotion, with a deviation from Figure 11. Path of the robot running over horizontal wood surface at straight of less than one degree. The robot can achieve a 360 different sprawl angles. degree rotation in less than one second by driving one side at

maximum speed and the other at zero. This puts the rotation center at the middle leg of the static side, and the turning radius is roughly 9 cm.

3) Running over stones and obstacles The robot successfully climbed over a rock bed including obstacles. The robot was able to successfully surmount a rocky incline, traversing a hill with a 30o slope while maintaining its orientation using the steering control.

Figure 12. The average normal impact force and standard deviation as function of the sprawl angle.

C. Robot Performance Maneuvers In this section we present various novel maneuvers that were performed with the robot.

1) Crawling over and under objects The minimum height of STAR, which is achieved at a sprawl angle of zero degrees, is 25mm. At this angle, the robot cannot advance. Increasing the sprawl to a few degrees, the Figure 14. The robot over a rock bed. robot remains practically flat due to its compliance but the ground contact on the external swing of the legs is favored 4) Negative sprawl angle and inverted running and the robot can advance. This allows the robot to crawl As previously mentioned, STAR reverses its motion under gaps of approximately 25o. When the legs are upright direction when the sprawl angle becomes negative (Figure 15

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